Transmission for driving counter-rotating propellers, lubrication system, and associated methods

ABSTRACT

A hybrid gear/sprocket-based transmission is provided for driving a pair of coaxial, counter-rotating propellers in aircraft and airboats. A drive shaft couplable to an engine crank shaft extends rearward into the transmission case, and a pair of coaxial hollow driven shafts extend rearward out of the transmission case, to which are attachable a pair of propellers. A gear train, containing an even number of gears, reverses the rotational direction of the engine; a sprocket train, containing two sprockets and a chain connecting them, retains the rotational direction of the engine. Improved stability characteristics are imparted by supporting the drive shaft at two points within the case and also by positioning the drive and the driven shafts in vertical alignment. The adaptability of the transmission to being coupled with an automobile engine confers improved noise, reliability, and fuel efficiency characteristics.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from provisional application No.60/177,051, “Transmission for Driving Counter-Rotating Propellers,Lubrication System, and Associated Methods,” filed Jan. 20, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to propulsion mechanisms for vesselsdriven by propellers, and, more particularly, to transmissions foraircraft and airboats, and to lubrication systems therefor.

2. Description of Related Art

Transmissions for driving a pair of counter-rotating propellers areknown in the art for both airplanes and airboats. Such transmissions areknown to be driven by belts and by gears, with input typically providedby a shaft driven by an aircraft engine operating at approximately2500-3000 revolutions per minute (rpm). Aircraft engines are extremelyexpensive, noisy, and fuel inefficient.

Airboat systems that utilize belt-driven transmissions are inefficientowing to power losses caused by belt friction, especially at higherrotational velocities. Belt breakage in these systems is a source offailure. Another disadvantage of belt-driven systems is their inabilityto permit reduction of engine speed, since the shaft used to effect sucha reduction would have to be too small to be practicable. Thus it hasbeen deemed advantageous to utilize a different transmission method inan airboat to enable engine speed reduction without loss of efficiency.

A belt-driven, two-engine counter-rotating system has been disclosed byStewart (U.S. Pat. No. 4,426,049). It has been taught by Becker et al.(U.S. Pat. No. 4,932,280) to use coaxial drive shaft systems for drivingmultiple outputs from a single input in an aircraft. Gearing means aredisclosed for driving two outputs at different speeds.

A double-sprocket and wheel transmission for driving multiple propellersin the same direction is described by Fay (U.S. Pat. No. 1,329,387).

The use of a gear-based transmission for airboats has been taught byKaye (U.S. Pat. No. 5,807,149), including a transmission for driving apair of counter-rotating coaxial shafts, to each of which is affixed apropeller. Such an arrangement can be used with an automobile engine,which is far more economical than an aircraft engine. This transmissionhas been shown to reduce noise and torque, to permit varying gearratios, to increase fuel efficiency and engine life, and to be lessexpensive to operate.

Improved gear-based transmissions for airboats have also been disclosedby Jordan (U.S. Pat. Nos. 5,724,867 and 6,xxx,xxx, the entire contentsof both of which are incorporated herein by reference). In the 'xxxpatent a lubrication system is also taught that includes a gear fordriving lubricant from a well to the interior of the inner output shaft,out of that space to a pair of stiffener bearings, and into the spacebetween the output shafts.

Another source of failure in transmissions is failure of the oil pumpthat heretofore has been considered an essential element.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a transmission thathas improved strength and stability characteristics for driving a pairof counter-rotating propellers.

It is a further object to provide such a transmission having a singleinput shaft for driving means for driving the two output driven shafts.

It is another object to provide a transmission for driving coaxialcounter-rotating propellers that is drivable at variable or equal speedsas desired.

It is an additional object to provide such a transmission with which itis possible to drive the counter-rotating propellers at different speedsto provide additional thrust, reduce noise output, and improve fuelefficiency.

It is also an object to provide such a transmission that is considerablylighter than previously known devices.

A further object is to provide a system and method for lubricating theoutput driven shafts that reduces weight and complexity.

Another object is to provide a system and method for achieving asimplified lubrication of a transmission and propulsion for an airboat.

An additional object is to provide such a system and method thateliminates the need for an oil pump to circulate lubricant.

These and other objects are achieved by the transmission of the presentinvention, which is for driving a pair of coaxial, counter-rotatingpropellers for, for example, an airboat or an aircraft, although theseare not intended as limiting usages. The transmission is housed in acase that has an interior space.

A drive shaft extends from the outside of the case into the interiorspace and is rotatable in a first direction. When the transmission is inuse on an aircraft or airboat, the drive shaft is mated at one end to amotor crank extending from and rotated by an engine. As mentioned above,previously known aircraft and airboats have utilized aircraft-typeengines; however, with the transmission of the present invention, it hasbeen found that an automobile engine can be used. This has a benefit ofreducing cost, as well as other benefits to be discussed below.

A first driven shaft also extends into the interior space of the case,typically from a side opposite that of the drive shaft. The first drivenshaft is for rotating an outer propeller, that is, the propeller fartherfrom the airboat body.

A second driven shaft, which is hollow, likewise extends into theinterior space of the case and is further positioned in surrounding,generally coaxial arrangement to the first driven shaft. The seconddriven shaft is shorter than the first, and both ends protrude beyondthe ends of the first driven shaft. This second driven shaft is forrotating an inner propeller, that is, the propeller closer to theairboat body.

A gear train for driving the first shaft is housed in the interior spaceof the case. In its simplest configuration, the gear train comprises twogears: a drive gear and a driven gear. The drive gear is coaxiallyaffixed to the drive shaft. The driven gear is coaxially affixed to thefirst shaft in such a position and configured so as to be rotatable bythe drive gear. Thus, when the drive shaft rotates in the firstdirection, the drive gear is rotated in the first direction. This causesthe first driven gear to be rotated in a second direction opposite insense to the first direction, which consequently drives the first shaftin the second direction.

In an alternate embodiment, additional intermediate driven gears may beinterposed between the drive gear and the first driven gear, so long asthe total number of intermediate gears is an even number.

A sprocket train is also housed in the interior space of the case. Thissprocket train includes a first and a second sprocket and a chain. Thefirst sprocket is coaxially affixed to the drive shaft. The secondsprocket is coaxially affixed to the first shaft. The chain is inengagement with both sprockets and is in such a position and configuredso as to rotate the second sprocket upon the first sprocket turning.Thus, when the drive shaft rotates in the first direction, the firstsprocket is rotated in the first direction. This causes the secondsprocket also to be rotated in the first direction, which consequentlydrives the first shaft in the first direction.

Thus it can be seen that the rotation of the drive shaft in onedirection achieves, through the action of the hybrid transmissioncomprising the gear train and the sprocket train, a counter-rotation ofthe two coaxial driven shafts and thus imparts counter-rotation topropellers attached thereto. There is no known system that uses fewercomponents than that of the present invention, which permits lowerweight, improved efficiency, and enhanced reliability.

Using the present hybrid gear/sprocket transmission permits driving anautomobile engine at the point of maximum horsepower, which generallyimplies a motor crank rotational speed approximately in the range of5000-5200 rpm, and then gearing down the rotational speed to roughly1200-2800, possibly even lower, a quieter speed at which to run thepropellers.

The invention is not, of course, limited to the use of an automobileengine; in fact, the presence of the gear and sprocket trains enablesthe user to optimize for efficiency and noise characteristics byaltering gear ratios as desired. An aircraft engine may also be used.

Yet another feature of the present invention is an improved lubricationsystem, in which an oil pump is no longer necessary, owing to thepresence of the two coaxial driven shafts, between which lubricant maypass and be moved by the counter-rotation thereof. This feature of thesystem comprises means for injecting a lubricant into a space betweenthe driven shafts and means for blocking the entry hole during use toretain the lubricant therein.

Among the benefits of the present invention are a minimization ofcomponents, which is believed to increase reliability and dependability,and a decrease in the weight, which increases performance and fuelefficiency. Present counter-rotator transmissions known in the art havea weight of 140-200 lbs, whereas the hybrid transmission of the presentinvention has a weight in the range of 90-110 lbs.

The features that characterize the invention, both as to organizationand method of operation, together with further objects and advantagesthereof, will be better understood from the following description usedin conjunction with the accompanying drawing. It is to be expresslyunderstood that the drawing is for the purpose of illustration anddescription and is not intended as a definition of the limits of theinvention. These and other objects attained, and advantages offered, bythe present invention will become more fully apparent as the descriptionthat now follows is read in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side cross-sectional view of a first embodiment of atransmission for driving counter-rotating propellers, also showing theshaft lubrication system.

FIG. 2 is a side cross-sectional view of a second embodiment of atransmission for driving counter-rotating propellers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description of the preferred embodiments of the present invention willnow be presented with reference to FIGS. 1 and 2.

The hybrid transmission 10 of the present invention, shown from the sidein FIG. 1, which is designed to drive a pair of coaxial,counter-rotating propellers 20 and 30, comprises a case 50 that has aninterior space 502, a fore side 504, and an aft side 506. It ispreferred that the case exterior be aerodynamically shaped in order toconfer good airflow characteristics to the transmission 10 during use athigh speeds.

A drive shaft 12 extends into the interior space 502 of the case 50through the case's fore side 504. The drive shaft 12 is rotatable in afirst direction, shown here as counterclockwise when viewed from thefront. Typically the drive shaft's fore portion 122, which extendsoutside the case 50, contains a coupler 124 for mating with a crankshaft 62 from an engine 60, which generates the rotational motion. Thedrive shaft 12 is preferably configured as a through shaft with respectto the case 50, and is thus supportable via brackets 508,509 affixed onthe inside of both the fore 504 and the aft 506 sides, respectively, ofthe case's interior space 502. This dual support confers exceptionalstability to the drive shaft 12.

The outer propeller 20 is mounted via propeller mount 202 adjacent theaft end 224 of, and is rotated by, a first driven shaft 22 that extendsfrom the back side 506 into the interior space 502 of the case 50. Thefore end 222 of the outer propeller 20 is supported via bracketing 510on the inside of the case's fore side 504. Preferably the first drivenshaft 22 comprises a hollow shaft, having a lumen 221 therethrough. Aremovable cap 223 at the aft end 224 leads to the lumen 221, as does ahole 225 adjacent the fore end 222.

The inner propeller 30 is mounted via propeller mount 302 to the aftportion 324 of, and is rotated by, a second hollow driven shaft 32 thatextends from the aft side 506 into the interior space 502 of the case50. The second hollow shaft 32 is positioned in surrounding, generallycoaxial arrangement to the first hollow driven shaft 20 and is shorterthan the first driven shaft 22. These relative lengths permit the foreend 222 and the aft portion 224 of the first driven shaft 22 toprotrude, respectively, beyond the fore end 322 and the aft portion 324of the second driven shaft 32. The second driven shaft 32 is supportedon the interior of the case's aft side 506 by bracketing 511.

In a preferred embodiment the longitudinal axes of the drive shaft 12and the first 22 and second 32 hollow driven shafts are positionedgenerally in vertical alignment. This positioning confers improvedstability during use, as the gyroscopic forces balance optimally in thisconfiguration.

The transmission 10 of the present invention further comprises two drivetrains housed within the case 50, one for driving each of the hollowdriven shafts 22,32. The first train 40 comprises a gear traincomprising an even number of gears for changing the incoming rotationaldirection. The embodiment shown in FIG. 1 contains two gears: a drivegear 402 coaxially affixed to the drive shaft 12 and a driven gear 404coaxially affixed to the first hollow shaft 22. The driven gear 404 ispositioned and configured so as to be rotatable by the drive gear 402.Thus, when the drive shaft 12 rotates in the first direction, here shownas counterclockwise, the drive gear 402 is rotated in the samedirection, and the driven gear 404 is rotated in a second directionopposite in sense to the first direction, that is, clockwise. Thus thefirst hollow shaft 22 is driven in a clockwise direction also, as wouldbe an attached propeller 20.

The second drive train 42 comprises a sprocket train for maintaining theincoming rotational direction. The sprocket train 42 comprises a drivesprocket 422, a driven sprocket 424, and a chain 426. The drive sprocket422 is coaxially affixed to the drive shaft 12, and the driven sprocket424 is coaxially affixed to the second hollow shaft 32. The chain 426 ispositioned in encompassing relation to the teeth of the sprockets422,424 and is configured so as to rotate the driven sprocket 424 upon arotation of the drive sprocket 422. Therefore, in use, when the driveshaft 12 rotates in the first direction, the drive sprocket 422 isrotated in the first direction, the driven sprocket 424 is also rotatedin the first direction, and the second hollow shaft 32 is rotated in thefirst direction, conferring counter-rotational movement to the innerpropeller 30 with respect to the outer propeller 20.

In an alternate embodiment 10′ (FIG. 2), the gear train 40 and thesprocket train 50 are interchanged in axial position, with the geartrain 40 driving the second hollow driven shaft 22 and the sprockettrain 50 driving the first hollow driven shaft 32.

Particular benefits of the hybrid propulsion system of the presentinvention include the possibility of using an even number of gears,since a planetary, intermediate, sense-changing gear is no longernecessary to achieve counter-rotation. In addition to the weight andcommensurate efficiency advantage conferred thereby, stability is alsoimproved, with balancing force vectors conferring added reliability anddurability. For example, failures are known to have been caused byoutward-pointing forces imposed upon the planetary gear by the drivingand driven gears. Further, all the gears in the present invention aremounted on through shafts, which are also stronger and more stable.

In either of the above-detailed embodiments it may be seen that thefirst and the second drive trains can be adapted to drive the propellersat different speeds, if desired which can provide improved thrustcharacteristics, increased fuel efficiency, and reduced noise.

Shaft Lubrication System

An additional aspect of the present invention comprises a lubricationsystem for delivering lubricant to elements of the propulsion system. Aparticular embodiment of the lubrication system, illustrated in FIG. 1,comprises means for injecting, sealing, and circulating a lubricantwithin an enclosed space including the hollow shafts 22,32 without theuse of a mechanical pump. Upon removal of the cap 223, lubricant may beinjected into the inner shaft's lumen 221. The rotational motion of theshaft 22 drives lubricant via “centrifugal force” from the inner shaft'slumen 221 out through a plurality of holes 228 in the aft portion of theshaft 22 to enter the space between the shafts 22,32, where there arepositioned a plurality of floating cylindrical bearings 66, whichmaintain the distance between the shafts 22,32 and also assist todistribute lubricant. In a preferred embodiment there are between twoand four of these bearings 66 positioned in spaced relation from eachother between the shafts 22,32, and the material comprises brass.Although brass is disclosed herein, it will be understood by one ofskill in the art that another material may be used, preferably a metaldissimilar from the material of which the shafts 22,32 are composed.

In order to provide a path for the escape of trapped air in the space tobe lubricated, a toroidal collar 44 is provided that is positionedaround the inner shaft 22 between the gear 404 and the bearing 510. Toan upper end of the collar 44 is affixed a line 45 that leads at a topend from the case's 50 top side 512, and is sealable with a removablecap 46. The line 45 leads at a top end to a void 441 in the collar 44that surrounds the inner shaft 22. Two 0-rings 47 are positioned insurrounding relation to the collar 44 around the inner shaft 22 toprevent lubricant from leaking out from the void 441 to the exterior ofthe inner shaft 22. Thus, in use, lubricant that has been injected intothe inner shaft's lumen 221 also proceeds forward through the lumen 221and reaches the collar's void 441. Any air bubbles that have beentrapped along the way can then escape through the uncapped line 45, andthen the cap 46 can be replaced. The collar 44 does not turn with theshaft 22, but rather floats thereon, being held substantially in placeby the line 45.

The bearings 66 themselves represent a novel lubrication element, beingdesigned to maximize lubricant return in the fore direction. In apreferred embodiment each bearing 66 has a series of generally helicalgrooves 662 cut in the outer surface, through which the lubricant maymove to be distributed within the outer shaft 32.

This lubrication system eliminates the oil pump and associated gearpreviously known and used in the art, and thus also improves fuelefficiency by reducing weight by 15-20 lbs.

It may be appreciated by one skilled in the art that additionalembodiments may be contemplated, including variable numbers and sizes ofgears, which may be positioned and configured to permit variablerelative speeds of the two counter-rotating propellers.

In the foregoing description, certain terms have been used for brevity,clarity, and understanding, but no unnecessary limitations are to beimplied therefrom beyond the requirements of the prior art, because suchwords are used for description purposes herein and are intended to bebroadly construed. Moreover, the embodiments of the apparatusillustrated and described herein are by way of example, and the scope ofthe invention is not limited to the exact details of construction.

Having now described the invention, the construction, the operation anduse of preferred embodiment thereof, and the advantageous new and usefulresults obtained thereby, the new and useful constructions, andreasonable mechanical equivalents thereof obvious to those skilled inthe art, are set forth in the appended claims.

What is claimed is:
 1. A method for making a transmission comprising thesteps of: rotatably affixing a driven sprocket having teeth within ahousing and to an inner shaft for rotating an outer propeller; rotatablyaffixing an aft driven gear within the housing and to a hollow outershaft coaxial with the inner shaft for rotating an inner propeller, theaft driven gear coaxial with the driven sprocket; opposedly affixing afore end and an aft end of a drive shaft for rotation within thehousing; mounting a drive sprocket on the drive shaft in drivingrelation to the driven sprocket; mounting an aft drive gear on the driveshaft in driving relation to the aft driven gear; meshing a chain withthe teeth of the driven sprocket and the drive sprocket; and providingmeans for driving the driven sprocket and the aft driven gear incounter-rotation.
 2. A transmission for converting an engine drive shaftrotation into coaxial counter-rotations for driving a pair ofpropellers, the transmission comprising: a fore driven gear havingteeth, the fore driven gear affixable to an inner shaft for rotating anouter propeller; a driven sprocket having teeth affixable to a hollowouter shaft coaxial with the inner shaft for rotating an innerpropeller, the driven sprocket coaxial with the fore driven gear; adrive shaft having an aft end rotatably affixable within the housing anda fore end having means for being rotatably driven; a fore gear affixedfor corotation with the drive shaft and having fore gear teethpositioned for meshing with the fore driven gear teeth for driving thefore driven gear in a first direction; a drive sprocket affixed forcorotation with the drive shaft and having for driving the drivensprocket in a second direction opposite the first direction; and a chainmeshable with the teeth of the driven sprocket and the drive sprocket.3. The transmission recited in claim 2, further comprising intermediategear shaft fore and aft bearing means for rotatably affixing a fore endand the aft end of the intermediate gear shaft to the housing,respectively.
 4. The transmission recited in claim 2, further comprisinginner shaft bearing means for rotatably affixing a fore end of the innershaft to the housing.
 5. The transmission recited in claim 2, whereinthe drive shaft fore end is matable with a crank shaft of an engine. 6.The transmission recited in claim 2, further comprising outer shaftbearing means for rotatably affixing an aft end of the outer shaft tothe housing.
 7. The transmission recited in claim 2, wherein the driveshaft and the inner and the outer shafts are positionable within thehousing in generally vertical alignment.
 8. A method for improving anefficiency and lowering a noise output of acounter-rotating-propeller-driven vehicle comprising the steps of:providing a propulsion system comprising: an inner shaft having meansfor affixing an outer propeller thereto; a hollow outer shaft having alumen therethrough and means for affixing an inner propeller thereto andpositionable in surrounding and coaxial relation to the inner shaft; afore driven gear having teeth, the fore driven gear affixable to theinner shaft; a driven sprocket having teeth affixable to the outershaft, the driven sprocket coaxial with the fore driven gear; a driveshaft having an aft end rotatably affixable within the housing and afore end having means for being rotatably driven; a fore gear affixedfor corotation with the drive shaft and having fore gear teethpositioned for meshing with the fore driven gear teeth for driving thefore driven gear in a first direction; a drive sprocket affixed forcorotation with the drive shaft and having teeth for driving the drivensprocket in a second direction opposite the first direction; a chainmeshable with the teeth of the driven sprocket and the drive sprocket;and utilizing the propulsion system to drive a pair of propellers incounter-rotating motion.
 9. A propulsion system for driving a pair ofpropellers in counter-rotation, the transmission comprising: an innershaft having means for affixing an outer propeller thereto; a hollowouter shaft having a lumen therethrough and means for affixing an innerpropeller thereto and positionable in surrounding and coaxial relationto the inner shaft; a fore driven gear having teeth, the fore drivengear affixable to the inner shaft; a driven sprocket having teethaffixable to the outer shaft, the driven sprocket coaxial with the foredriven gear; a drive shaft having an aft end rotatably affixable withinthe housing and a fore end having means for being rotatably driven; afore gear affixed for corotation with the drive shaft and having foregear teeth positioned for meshing with the fore driven gear teeth fordriving the fore driven gear in a first direction; and a drive sprocketaffixed for corotation with the drive shaft and having teeth for drivingthe driven sprocket in a second direction opposite the first direction;and a chain meshable with the teeth of the driven sprocket and the drivesprocket.
 10. A transmission for converting an engine drive shaftrotation into coaxial counter-rotations for driving a pair ofpropellers, the transmission comprising: a housing having an interiorspace, a fore side, and an aft side opposed to the fore side, the foreside affixable to an engine bell housing, an opening in the aft side forpermitting a pair of coaxial propeller shafts to pass therethrough, andan opening in the fore side; a fore driven gear having teeth, the foredriven gear positioned within the housing interior space and affixableto an inner shaft for rotating an outer propeller; a driven sprockethaving teeth positioned within the housing interior space and affixableto a hollow outer shaft coaxial with the inner shaft for rotating aninner propeller, the driven sprocket coaxial with the fore driven gear;a drive shaft having an aft end rotatably affixable within the housingand a fore end extending through the housing fore side opening andhaving means for being rotatably driven by an engine; a fore gearaffixed for corotation with the drive shaft and having fore gear teethpositioned for meshing with the fore driven gear teeth for driving thefore driven gear in a first direction; a drive sprocket affixed forcorotation with the drive shaft and having teeth for driving the drivensprocket in a second direction opposite the first direction; and a chainmeshable with the teeth of the driven sprocket and the drive sprocket.11. The transmission recited in claim 10, further comprising bearingmeans for rotatably affixing: the inner shaft to the housing fore side;the outer shaft to the housing aft side; and the drive shaft to thehousing fore side and aft side.
 12. A propulsion and lubrication systemcomprising: a hollow inner shaft for rotating an outer propeller havingan opening at an end thereof and a lumen therethrough in communicationwith the hole; a hollow outer shaft coaxial with the inner shaft forrotating an inner propeller positioned in surrounding and coaxialrelation to the inner shaft; removable means for sealing the opening inthe inner shaft; means for driving the inner and outer shaft incounter-rotating relation; and bearing means positionable between theinner and the outer shafts in covering relation to the hole, the bearingmeans movable relative to the inner shaft; wherein lubricant is sealablewithin the inner shaft and is distributable through the hole, betweenthe inner shaft and the bearing means, into a space between the innerand the outer shaft, without a pump for circulating the lubricant. 13.The propulsion and lubrication system recited in claim 12, wherein thebearing means comprise a helical groove in an outer surface thereof. 14.The propulsion and lubrication system recited in claim 12, wherein theinner and the outer shaft comprise a first material and the bearingmeans comprises a second material distinct from the first material. 15.The propulsion and lubrication system recited in claim 14, wherein thesecond material comprises brass.
 16. The propulsion and lubricationsystem recited in claim 12, wherein the bearing means comprises asubstantially cylindrical element slidable over the inner shaft.
 17. Thepropulsion and lubrication system recited in claim 16, wherein the holecomprises a plurality of sets of holes and the cylindrical elementcomprises a plurality of cylinders positioned in axially spaced relationalong the inner shaft, each of the plurality of cylinders in coveringrelation to a one of the plurality of sets of holes.
 18. A method formaking a transmission comprising the steps of: rotatably affixing a foredriven gear having teeth within a housing and to an inner shaft forrotating an outer propeller; rotatably affixing a driven sprocket havingteeth within the housing and to a hollow outer shaft coaxial with theinner shaft for rotating an inner propeller, the driven sprocket coaxialwith the fore driven gear; opposedly affixing a fore end and an aft endof a drive shaft for rotation within the housing; mounting a fore drivegear on the drive shaft in driving relation to the fore driven gear;mounting a drive sprocket on the drive shaft in driving relation to thedriven sprocket; meshing a chain with the teeth of the driven sprocketand the drive sprocket; and providing means for driving the fore drivengear and the driven sprocket in counter-rotation.
 19. A transmission forconverting an engine drive shaft rotation into coaxial counter-rotationsfor driving a pair of propellers, the transmission comprising: a drivensprocket having teeth affixable to an inner shaft for rotating an outerpropeller; an aft driven gear having teeth, the aft driven gearaffixable to a hollow outer shaft coaxial with the inner shaft forrotating an inner propeller, the aft driven gear coaxial with the drivensprocket; a drive shaft having an aft end rotatably affixable within thehousing and a fore end having means for being rotatably driven; a drivesprocket affixed for corotation with the drive shaft and having teethfor driving the driven sprocket in a first direction; an aft drive gearaffixed for corotation with the drive shaft and having aft gear teethpositioned for meshing with the aft driven gear teeth, the aft gearteeth for driving the aft driven gear in a second direction opposite thefirst direction; and a chain meshable with the teeth of the drivensprocket and the drive sprocket.
 20. A propulsion system for driving apair of propellers in counter-rotation, the transmission comprising: aninner shaft having means for affixing an outer propeller thereto; ahollow outer shaft having a lumen therethrough and means for affixing aninner propeller thereto and positionable in surrounding and coaxialrelation to the inner shaft; a driven sprocket having teeth affixable tothe inner shaft; an aft driven gear having teeth, the aft driven gearaffixable to the outer shaft, the aft driven gear coaxial with thedriven sprocket; a drive shaft having an aft end rotatably affixablewithin the housing and a fore end having means for being rotatablydriven; a drive sprocket affixed for corotation with the drive shaft andhaving teeth for driving the driven sprocket in a first direction; andan aft gear affixed for corotation with the drive shaft and having aftgear teeth positioned for meshing with the aft driven gear teeth fordriving the aft driven gear in a second direction opposite the firstdirection; and a chain meshable with the teeth of the driven sprocketand the drive sprocket.
 21. A transmission for converting an enginedrive shaft rotation into coaxial counter-rotations for driving a pairof propellers, the transmission comprising: a housing having an interiorspace, a fore side, and an aft side opposed to the fore side, the foreside affixable to an engine bell housing, an opening in the aft side forpermitting a pair of coaxial propeller shafts to pass therethrough, andan opening in the fore side; a driven sprocket having teeth positionedwithin the housing interior space and affixable to an inner shaft forrotating an outer propeller; an aft driven gear having teeth, the aftdriven gear positioned within the housing interior space and affixableto a hollow outer shaft coaxial with the inner shaft for rotating aninner propeller, the aft driven gear coaxial with the driven sprocket; adrive shaft having an aft end rotatably affixable within the housing anda fore end extending through the housing fore side opening and havingmeans for being rotatably driven by an engine; a drive sprocket affixedfor corotation with the drive shaft and having teeth for driving thedriven sprocket in a first direction; an aft drive gear affixed forcorotation with the drive shaft and having aft gear teeth positioned formeshing with the aft driven gear teeth for driving the aft driven gearin a second direction opposite the first direction; and a chain meshablewith the teeth of the driven sprocket and the drive sprocket.
 22. Amethod for improving an efficiency and lowering a noise output of acounter-rotating-propeller-driven vehicle comprising the steps of:providing a propulsion system comprising: an inner shaft having meansfor affixing an outer propeller thereto; a hollow outer shaft having alumen therethrough and means for affixing an inner propeller thereto andpositionable in surrounding and coaxial relation to the inner shaft; adriven sprocket having teeth affixable to the inner shaft; an aft drivengear having teeth, the aft driven gear affixable to the outer shaft, theaft driven gear coaxial with the driven sprocket; a drive shaft havingan aft end rotatably affixable within the housing and a fore end havingmeans for being rotatably driven; a drive sprocket affixed forcorotation with the drive shaft and having teeth for driving the drivensprocket in a first direction; an aft gear affixed for corotation withthe drive shaft and having aft gear teeth positioned for meshing withthe aft driven gear teeth for driving the aft driven gear in a seconddirection opposite the first direction; and a chain meshable with theteeth of the driven sprocket and the drive sprocket; and utilizing thepropulsion system to drive a pair of propellers in counter-rotatingmotion.